A conventional variable valve mechanism is disclosed. The conventional variable valve mechanism is capable of changing the lift amount of a valve disc that opens/closes in synchronism with camshaft rotation. The variable valve mechanism disclosed, for instance, by Japanese Patent Laid-Open No. Hei 7-63023 is capable of changing the lift amount of a valve disc in accordance with the rotation position of an eccentric shaft. In this variable valve mechanism, a compression spring (lost motion spring) is used to push a rocker lever, which is provided with a roller, in order to ensure that a roller whose contact with a cam varies in position with the eccentric shaft rotation position is pressed against the cam. When this variable valve mechanism is employed, the compression spring works to ensure that the cam is in mechanical contact with the roller at all times.
In the conventional mechanism disclosed by Japanese Patent Laid-Open No. Hei 7-63023, however, the compression spring coordinates with a valve spring to press the roller toward the cam. As a result, the eccentric shaft receives a force that is applied in a fixed direction. Consequently, the required drive torque of an actuator for eccentric shaft rotation increases, thereby lowering the responsiveness of a variable valve or increasing the power consumption.
Another variable valve mechanism disclosed, for instance, by Japanese Patent Laid-Open No. Hei 7-293216 is capable of changing the lift amount of a valve disc of an internal combustion engine. This variable valve mechanism includes a mechanical device that is positioned between the valve disc and cam to change the lift amount. This mechanical device increases the lift amount of the valve disc when a control shaft rotates in a certain direction, and decreases the lift amount of the valve disc when the control shaft rotates in another direction. When this mechanical device is employed, the lift amount of the valve disc can be arbitrarily changed by rotating the control shaft as appropriate.
The valve disc of an internal combustion engine is generally provided with a valve spring, which pushes the valve disc in the valve closing direction. Therefore, when the conventional variable valve mechanism opens the valve disc, the valve spring's reactive force is exerted on the mechanical device between the valve disc and cam. The greater the lift amount for the valve disc, the greater the reactive force.
The mechanical device described above is dynamically stabler when the reactive force exerted on the mechanical device is small than when the reactive force exerted on the mechanical device is increased with an increase in the lift of the valve disc. Therefore, the mechanical device is generally likely to change its state to decrease the lift amount. In other words, it is likely that a reactive force for changing the mechanical device state to a state corresponding to a small lift will be transmitted to the above-mentioned control shaft.
If the above reactive force is transmitted to the control shaft to change the control shaft status, an appropriate lift amount cannot be maintained for the valve disc. Therefore, this type of variable valve mechanism needs a mechanism for maintaining the control shaft status constant without regard to the valve spring's reactive force.
The control shaft of the conventional variable valve mechanism disclosed by Japanese Patent Laid-Open No. Hei 7-293216 is driven by a motor via a gear mechanism. This gear mechanism includes a worm gear, which is installed over a motor rotation shaft, and a worm wheel, which meshes with the worm gear. The gear mechanism, which includes the worm gear and worm wheel, provides high normal efficiency and low inverse efficiency due to a great friction force exerted between the worm gear and worm wheel and a great gear ratio between them.
The above gear mechanism makes it possible to transmit a motor-generated torque to the control shaft with high efficiency and properly prevent the input to the control shaft from being transmitted to the motor. Therefore, the above conventional variable valve mechanism can accurately control the control shaft status without being affected by the valve spring. As a result, it is possible to accurately control the lift amount of the valve disc.
However, when the lift amount of the valve disc in the conventional variable valve mechanism disclosed by Japanese Patent Laid-Open No. Hei 7-293216 is to be increased, it is necessary to rotate the control shaft in resistance to a reactive force for decreasing the lift amount. More specifically, it is necessary to rotate the control shaft in the direction of increasing the lift amount in resistance to the valve spring's reactive force for decreasing the lift amount.
To meet the above requirements, it is necessary that the motor generate a great driving force. As a result, a motor cost increase, power consumption increase due to motor use, motor mountability deterioration due to structural expansion, and various other problems arise. Further, if such a great force is exerted on the control shaft, the control shaft may significantly become distorted. In addition, the transmission of such a great force increases the gear-to-gear contact load, thereby accelerating the wear of gears.
The present invention has been made to solve the above problems. It is an object of the present invention to provide an internal combustion engine variable valve mechanism for changing the lift amount and operating angle of a valve that opens/closes in synchronism with camshaft rotation, and reduce the required load on a variable valve.